The code attribute gives the
code being defined. We use the XSchema
“ID” datatype
because this element defines this location code as a
unique identifier so that other attributes can refer
to it. It must be a valid XML name, starting with a
letter and consisting only of letters, digits,
underbar, period, and colon. Example:

code='BdA'

Originally, this field was defined as a unique
identifier of type xsd:ID. However,
this won't work, because ID values
must be unique within a file, and in our case they
must be unique only within a day-notes
element. This problem was caught by the xmllint verifier using the Relax NG version
of this schema:

Following any gps element
children, you can include arbitrary text as necessary
to describe other relevant information such as
directions for getting there, comments on habitat
change, local names, etc.